The present invention relates to the preparation of aluminum sheet material suitable for fabrication into can ends. In particular this invention relates to the preparation of can end stock from continuous chill roll cast sheet aluminum, and more particularly to the preparation of a continuous chill roll cast aluminum sheet suitable for subsequent fabrication in to aluminum can end stock.
Currently, the wide spread concern about the future availability of energy with the resultant concentration on energy conservation, particularly in the aluminum industry, has produced several innovations relating the effective utilization of container scrap as a suitable starting material for the subsequent fabrication of new containers, particularly beverage containers. Less energy is required using scrap as a starting material resulting in lower costs if container scrap containing both body stock and can end stock could be successfully used to make materials suitable for fabrication into new can bodies and can ends.
Typically, substantial modification of existing commercial practices used with can body and can end alloys for the preparation of sheet material from either direct cast or continuous casting processes are required before suitable can stock could be obtained from container scrap, and particularly before suitable can end stock can be obtained which incorporates easy opening features. Exemplary of these efforts are the processes disclosed in U.S. Pat. No. 3,787,248 to William C. Setzer, et. al. issued Jan. 22, 1974; U.S. Pat. No. 3,851,787 to William C. Setzer, et. al. issued Dec. 3, 1974; U.S. Pat. No. 3,802,931 to Linton D. Bylund issued Apr. 9, 1974, and the recent inventions of Robertson, et. al., U.S. patent and applications Ser. Nos. 931,041, 931,040 and 931,036 as well as U.S. Pat. No. 4,238,248 of Ivan Gyongyos, et. al. issued Dec. 9, 1980 and U.S. Pat. No. 4,235,646 of Kurt Neufeld, issued Nov. 25, 1980.
The foregoing patents variously disclose direct chill ingot cast and continuous block type casting processes for utilizing the specific compositions which would be encountered in alloys derived from aluminum scrap and in particular aluminum container scrap.
Moser, et. al., U.S. Pat. No. 3,930,895 issued Jan. 6, 1976, in an example of a process for making can body stock from continuous chill roll cast aluminum to improve the deep drawing characteristics of a modified body stock alloy.
U.S. Patent to J. L. Hunter, No. 2,790,216 issued Apr. 30, 1957, to J. L. Hunter discloses a conventional method and apparatus for continuously chill roll casting aluminum alloys which is incorporated herein by reference. The apparatus disclosed produces a chill cast product of sheet metal stock which is generally characterized by a uniform grain microstructure including particles of intermetallic compounds including a compound based on Al-Mn, dispersed throughout the alloy matrix.
It has been desireable to employ the Hunter Apparatus disclosed in U.S. Pat. No. 2,790,216 for the continuous chill roll casting of aluminum alloys. Difficulties are however encountered in producing satisfactory container end stock utilizing alloys derived from container scrap when using the Hunter type of process and apparatus. Can ends utilizing easy opening features, such as ring pull tabs and stay-on tabs for containers which must withstand at least 50 pounds per square inch internal pressure, require special physical properties in order to withstand the severe forming operations that are encountered in the fabrication of the easy opening feature.
It is therefore an objective of the present invention to provide a process for the production of highly formable continuous chill roll cast aluminum sheet stock from aluminum alloy compositions normally encountered in mixed container scrap. This sheet stock must exhibit an ability to be fabricated into can ends having easy open features.
It is a further object of the present invention to provide an aluminum sheet material which is characterized by a particular microstructure in an aluminum alloy which contains between 1.3% to 2.5% by weight magnesium and between 0.4% to 1.0% by weight manganese.